We report on power scaling of optically-pumped semiconductor disk lasers using multiple gain scheme. The method allows for significant power improvement while preserving good beam quality. Total power of over 8 W was achieved in dual-gain configuration, while one-gain lasers could produce separately about 4 W, limited by the thermal rollover of the output characteristics. The results show that reduced thermal load to a gain element in a dual-gain cavity allows extending the range of usable pump powers boosting the laser output.
©2006 Optical Society of America
Optically pumped semiconductor disk lasers (SEDLs), also known as vertical-external-cavity surface-emitting lasers (VECSELs), can produce high power and good quality of the beam , however, both parameters are critically dependent on the efficiency of the heat removal from the gain structure operating under strong pumping condition. Heat spreaders with high thermal conductance, e.g. diamond, SiC or copper, providing efficient heat dissipation, are used in high power SEDLs to reduce rollover and thermal lensing [2–3]. Although, technology of the wafer bonding and heat spreaders constantly improves, the beam quality and output powers achievable from SEDL would eventually be limited by the state-of-the-art of the thermal management. In order to increase the SEDL power, a number of approaches have been considered. The power scaling could obviously be achieved by increasing the mode size on the gain medium, however, some penalty to the beam quality can generally be expected . It is also possible to distribute the pump power into a number of spots over the gain structure forming arrayed or “parallel” geometry . The resulted increase in the output power would, however, correspond to the multiple beams with complicated spatial distribution. With gain segments formed on a large semiconductor chip and connected in series by the multibounce cavity, the fundamental-transverse mode with increased output power could be expected . This geometry, however, uses single heatsink, which may limit efficiency of heat extraction and is difficult to implement practically.
In this study we use two entirely separated gain elements with individual heat-spreaders placed in the same cavity for power scaling, while preserving high quality of the output beam. In principle, the concept can be applied to a number of gain elements. The multiple gain cavity could take on higher pump powers by sharing thermal load among different gain elements thus avoiding excessive heating and the rollover. It should be mentioned that similar approach was demonstrated to be attractive for solid-state disk lasers . The results show impressive power scalability up to 1 kW, however, with compromised beam quality.
2. Gain material and laser setup
The laser structure was designed for operation around 1050 nm and it comprises a 30.5-pair GaAs/AlGaAs distributed Bragg reflector and a gain section with 13 non-strain-compensated Ga0.74In0.26As quantum-wells grown monolithically on a GaAs substrate by molecular beam epitaxy. In order to improve the heat transfer from the gain section, transparent diamond heat spreaders of 300-µm thickness were capillary bonded on the top of the 2.5×2.5 mm gain samples. The gain samples were mounted in water-cooled copper heat sinks. The diamond-air interface was coated with a two-layer TiO2-SiO2 film to reduce pump and signal reflection. The Z-shaped laser cavity was defined by the two semiconductor gain mirrors, a curved 5%- output coupler mirror and a high reflective spherical folding mirror. The laser setup is shown in Fig 1. The gain structures were optically pumped at ~800 nm with multimode fiber-coupled diode systems at an angle of about 35°. The pump spot diameter on both gain media was about 180 µm, which matches the fundamental mode size of the cavity expected from numerical simulation. During all measurements, the temperature of the copper mounts was kept at 15°C.
The output characteristics, spectrum and beam quality factor M2 were measured from the dual-gain laser and two lasers each using single-gain element. The performance of each gain mirror was tested in a single-gain cavity with output coupling of 3%. The lasers were operated with the center wavelength near 1050 nm. Light output characteristics, shown in Fig. 2, demonstrate that power achieved from the dual-gain laser is increased by a factor of two compared with single-gain setup. This result indicates that the thermal load on the gain structures in dual-gain configuration is reduced compared with one-gain laser pumped with the same power. Consequently, the thermal rollover in a single-gain scheme limits usable pump power and prevents power scaling. In contrary, the dual-gain laser has an increased threshold of thermal rollover and allows for significant power scaling.
The beam quality factor M2 was measured for orthogonal directions using an automated scanning slit device. For single-gain lasers M2 parameter was ~1.2 in both directions at the maximum power. The dual-gain disk laser has good beam stability with the output power indicating only minor increase of the M2- factor by 15–20% up to 1.28 / 1.45 at the output power of 8 W. The spatial beam distribution fitted with Gaussian shape is shown in Fig. 3.
In conclusion, the motivation of this study is given by the ever growing pump power that cannot be entirely absorbed by a single gain element of the disk laser without an increase in the pump area. The solution based on the increase of the mode size is limited, since it is ultimately accompanied by certain degradation of the beam quality. We have demonstrated the method of power scaling in semiconductor disk lasers using dual-gain cavity. Dual-gain concept reduces the thermal load of the gain material, increases the threshold of rollover and extends capability for boosting the output power. Reduced thermal lensing also prevents the degradation in the beam quality. We have recognized that disk lasers could naturally adopt multi-gain cavity. With this geometry, the strong pump shared among gain media could be efficiently absorbed without thermal overload. The output from a high-power pump source can be split into a few beams to pump each gain structure separately. This geometry seems to be a promising solution for power scaling preserving the near diffraction-limited beam.
The authors acknowledge the support from the Academy of Finland, The Employment and Economic Development Centre for Pirkanmaa, EU FP6 NATAL (Contract No. 016769), Nokia foundation, Jenny and Antti Wihuri foundation and Emil Aaltonen’s foundation.
References and links
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